The obelisk outside St. Peter's Basilica in Rome serves as a gnomon, or pointer, on the most prominent sundial in Christendom. On the summer solstice, when the sun is at its highest point over Rome, the obelisk casts virtually no shadow. But every day before and after, shadows creep along a meridian line embedded in the pavement. Each month, the tip of the shadow rests on one of several tablets etched with signs of the zodiac. A scientific device planted within Vatican City might seem out of place at first. The Roman Catholic Church, after all, is the institution that savaged Galileo in the 17th century and only apologized for it three centuries later. So what is such a tool—with its reference to the sun and the planets—doing inside the famous piazza?
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